Here's the thing- let's say you've been doing a ton of pumping and get yourself into a place where you are making 3-4 oz an hour or something like that. Not a huge oversupply, right? Then you go back to work. And instead of nursing/pumping every 1-3 hours, you're routinely going 3 or more hours without removing milk. By the time you get to your pump, you're likely to be very uncomfortably full. And getting that full, that uncomfortable- that can lead to nasty things like engorgement, plugged ducts, mastitis... That's why moms are not encouraged to pump themselves into oversupply before a return to work, particularly if pumping opportunities at work are difficult to come by.
I would think that your supervisor would rather have you back in the office and taking more pump breaks than at home on FMLA trying to build a stash. I'd give her a call soon and discuss pumping options. No-one here wants you to have to compromise patient care, but we also don't want to see you needing to supplement with formula because your work doesn't accommodate normal human needs!